ABSTRACT: Lithologic and Geometric Variations in Stratigraphic Sequences across Coastal Plain, Shelf, and Slope Environments, Middle Eocene, Ventura Basin

CLARK, MICHAEL S., ARCO Oil and Gas Company, Bakersfield, CA

Discontinuity-bounded, reservoir-scale (6-45 m thick) stratigraphic sequences in the middle Eocene section of the northeastern Ventura basin, California, show variations in cycle symmetries, bounding surfaces, and facies successions across coastal plain, shelf, and slope environments. Coastal plain sequences are asymmetric fining-upward facies successions bounded by unconformities. Trough cross-stratified sandstones at the bases of sequences represent uppermost shoreface environments, and green to red siltstones at the tops of sequences represent brackish water to nonmarine environments. These sequences are replaced laterally in marine shelf environments by temporally equivalent flooding surfaces. Thus, the coastal plain sequences are aggradational units deposited as base level is ris ng.

Shelf sequences are asymmetric coarsening- and shallowing-upward facies successions bounded by marine flooding surfaces. Bioturbated marine mudstones at the bases of sequences grade upward into swaley and trough cross-stratified sandstones deposited in wave-dominated shorefaces. These sequences are replaced laterally in coastal plain environments by temporally equivalent unconformities. Thus, the shelf sequences are progradational units deposited as base level is falling.

Lower slope to basin sequences are asymmetric thickening- and coarsening-upward successions of graded mudstone that grade upward into interbedded mudstones and turbidite sandstones. Thus, these sequences are probably deposited as base level is falling.

A few symmetric units, with coarsening-upward bases and fining-upward tops, are present at coastal plain to shelf, and shelf to slope transitions. These units, by analogy to the two types of asymmetric sequences, represent base-level fall followed by base-level rise. Thus, stratigraphic sequences in different depositional settings, from coastal plain to shelf to slope, accumulate during different parts of the base-level transit cycle. Similar lithologic and geometric variations may be present in similar-scale sequences in other depositional settings.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)